1. Field of the Invention
The present invention relates to an electromagnetic induction type position sensor which detects the position of movable members using an electromagnetic induction function.
2. Description of the Related Art
Position sensors measuring magnetic properties to detect, for example, the position of a main shaft of a machine tool have heretofore been known. In a typical such position sensor, a scale, which is a cylinder made of a soft magnetic material whose outer periphery is indented, is fixed to a rotating shaft; any change in the reluctance of the scale outer periphery is converted into an electric signal; and a relative position is thereby detected. Because such a position sensor bases positional detection on magnetic properties, this type of sensor inherently excels in resistance to exposure to common environmental contaminants such as water, oil, etc. Such a position sensor also allows scales of different sizes to be easily manufactured by machining. Thus, there is a further advantage that it is not necessary to produce a different mold for each size.
A position sensor as described above is described in, for example, Japanese Patent Publication Laid-open No. 62-274215, which discloses a sensor wherein a permanent magnet is combined with a magnetoresistive element, and a reluctance change of the scale is detected as an electric signal based on a resistance change. Another sensor to detect a change of a magnetic flux due to the reluctance change of the scale is disclosed in Japanese Patent Publication Laid-open No. 5-180664 and Japanese Patent Publication Laid-open No. 8-21744. The sensor described therein utilizes an electromagnetic induction function, wherein a plurality of coils is formed by conductive patterns using, for example, a printed board technology, and these coils are used to detect the change of reluctance as an amplitude change of an AC magnetic flux. Further, Japanese Patent Publication Laid-open No. 8-313295 shows that the sinusoidal shape of the conductive pattern allows the amplitude change of an electromagnetic induction voltage output by the coil to be approximate to a sinusoidal wave with regard to the positional change. Still further, Japanese Patent Publication Laid-open No. 9-210611 discloses a method wherein the coil formed by the sinusoidal conductive pattern is an inwrought coil so that a plurality of coils is mounted at nearly the same location with a high density.
In the meantime, there has also recently been a marked increase in the use of machine tools performing a composite process. In the main shaft of a composite process machine, a process target fixed to the main shaft side is accurately rotated, and outline processing is performed with a rotating tool. Such a processing application requires that a rotational position of the main shaft or the like be accurately detected. However, the position sensor using the magnetoresistive element disclosed in Japanese Patent Publication Laid-open No. 62-274215, which is widely used as the position sensor for the main shaft, is not suitable for highly accurate position detection because of unsatisfactory temperature characteristics of the magnetoresistive element. Moreover, the use of the permanent magnet can be problematic in that iron powder produced, for example, by the machine work, tends to stick to sensor surfaces, where its presence reduces the accuracy of detection.
On the other hand, the position sensor using the plurality of coils as described above does not have the problem of sticking iron powder, and has significantly more stable temperature characteristics. Further, the sinusoidal shape of the coils makes it possible to obtain a signal amplitude change close to the sinusoidal shape for positional displacement. It is for this reason characterized in that the position in a scale indentation pitch can be detected with a high resolution by interpolation processing of the amplitude change of a plurality of detection signals. However, a problem with a sensor using a plurality of coils is that the unamplified output signal level is low, such that, if the indentation pitch of the scale is reduced for higher accuracy, the detectable signal level will be extremely low. Thus, a smaller indentation pitch of the scale has led to a lower signal-noise ratio. In addition, even with the reduced indentation pitch, detection resolution in the indentation pitch will decrease due to noise, so that it is difficult to increase the accuracy.
Furthermore, when, for example, printed circuit board technology is used to arrange the coils of the sinusoidal conductive patterns having a short wavelength, there is also a problem that an inwrought coil can not be implemented because for the footprint of an interlayer connection is several times larger than a conductor width (thickness of the conductive pattern). Thus, in order to implement the position sensor capable of handling the scale with a small indentation pitch, the plurality of coils must be arranged at different places. This has caused a problem that the area of the coils receiving the magnetic flux is reduced, leading to reduced detected signal level. Moreover, there is a problem that, although the inwrought coil can be implemented in the small indentation pitch, it is difficult to maintain the accuracy of the sinusoidal coil shape via a plurality of layers. Thus, there is also a problem that, even if the detected signal level can be increased by the inwrought coil, highly accurate positional detection in the indentation pitch becomes possible due to degraded coil shape accuracy.
In addition, as in Japanese Patent Publication Laid-open No. 8-313295, use is made of a coil in which two sinusoidal conductive patterns having a large amplitude in reverse phases are connected in series so as to perform highly accurate positional detection, in which case a coil and a scale are needed which generate an AC magnetic flux that is sufficiently great for the amplitude of the sinusoidal conductive patterns, resulting in a problem that the size of the position sensor is increased. The reason for this is that the farther the center where the magnetic flux is generated is, the less uniform the magnetic flux becomes in the coil generating the AC magnetic flux, so that if the coil to generate the AC magnetic flux is small, part of the sinusoidal conductive pattern is disposed even in a place where the magnetic flux change is great. Also, when the scale is small in a thickness direction, a portion of the sinusoidal conductive pattern is disposed at a location where the magnetic flux proximate to a scale edge rapidly changes, thus posing the same problem.
The present invention has been attained under such circumstances, and the present invention advantageously provides an electromagnetic induction type position sensor which is smaller and more accurate.